The motor trigeminal nucleus (MTN) of the rat and its afferent and efferent projections will be used as a model system to investigate the regulation of appropriate quantitative innervation patterns in the central nervous system. First, the immunocytochemical identity of MTN afferents, their distribution on the cell somata of masseter and temporalis trigeminal motoneurons and their patterns of development within the MTN will be determined. Second, types of afferent- and target-specific manipulations which have provided information regarding control of afferent density and innergation patterns in diverse systems will be carried out on the MTN system. The availability of monoaminergic afferents to the trigeminal motoneurons will be increased by neonatal administration of monoamine neurotoxins. Neonatal treatment with 6-hydroxydopamine has been demonstrted to cause a redistribution of morphologically distinct axosomatic terminals in the MTN, whereas the density of innervation of motoneuron somata remains constant. The number of available masseter motoneurons will be decreased by retrograde transport of doxorubicin, and fast axoplasmic transport in the axons of these motoneurons will be blocked by local application of colchicine in both neonates and adults. Following each of these manipulations, the distribution of immunocytochemically or morphologically distinct axosomatic terminals innervating the temporalis and masseter motoneurons will be determined and compared with controls. The degree of variability which can be tolerated by the system may provide clues as to the degree and type of control tht regulates these parameters during normal development. This information will be useful in understanding the pathophysiology of developmental disorders including motoneuron disease and mental retardation.